Particle size analysis through means of light obscuration or light extinction is often the preferred method of size determination, for several reasons. Light obscuration offers particle size distribution measurement as well as mean size assignment. Particle size measurements by light obscuration exhibit good reproducibility, relatively quick analyses, applicability in a wide range of fluids, and can be used in the analysis of particles in the size range of approximately 1 to 2500 .mu.m. Accordingly, light obscuration is a widely practiced technique for particle size determinations.
Particle sizing by light obscuration is based on the principle of light scattering. Generally, a light source is employed which delivers a collimated beam of light with a uniform intensity distribution to a photodetector. A flow cell allows particles suspended in a fluid medium to intercept the collimated beam of light. The fluid that carries the particles through the flow cell can be either a gas or a liquid. The portion of the collimated beam that intersects the fluid within the flow cell is referred to as the measurement volume. The amount of light incident on the detector in the absence of particles is constant and known. As a particle passes through the measurement volume, the particle will scatter light, resulting in a decrease of light incident on the detector. For any given particle and fluid medium composition, large particles will scatter more light than small particles. Accordingly, the reduction in the amount of light incident on the detector, observed as a particle passes through the measurement volume, is used to determine the size of the particle.
While this technique has proven to be very valuable, certain deficiencies still exist. One of these deficiencies is that the accuracy of the commercial sensors is dependent upon the relative refractive index of the particles and the fluid medium.
It is therefore an objective of the current invention to provide an instrument and a method for determining the size of particles suspended in a medium wherein the determination is less dependent of the relative refractive index of the materials.